The invention relates to a mixture which contains (a) a compound which can undergo free radical polymerization, contains at least two terminal ethylenically unsaturated groups and has a boiling point of above 100.degree. C. under atmospheric pressure, (b) a compound capable of initiating the polymerization of compound (a) under the action of actinic radiation, and (c) a water-insoluble copolymer which is soluble in aqueous alkaline solutions.
Polymerizable mixtures of the stated type are known, and are used, inter alia, for the preparation of photopolymerizable printing plates and photoresist materials. A preferred field of use for such mixtures is in the preparation of dry photoresist materials.
German Offenlegungsschrift No. 2,064,080 describes mixtures of this type that contain, as binders, copolymers of methacrylic acid and alkyl methacrylates having at least 4 carbon atoms in the alkyl group, preferably terpolymers of methacrylic acid, methyl or ethyl methacrylate and an alkyl methacrylate having 4 to 15 carbon atoms in the alkyl group. These mixtures form flexible layers which adhere well but which, when they are relatively thick, e.g., greater than 20 .mu.m, tend to exhibit cold flow and, on prolonged storage in the form of coated, rolled-up dry resist films, extrude at the edges of the roll and stick.
German Offenlegungsschrift No. 2,363,806 describes similar mixtures which contain terpolymers of methacrylic acid, an alkyl methacrylate having at least 4 carbon atoms in the alkyl group and a further monomer whose associated homopolymer has a glass transition temperature of at least 80.degree. C. Particular examples of such monomers are styrene, substituted styrenes and acrylonitrile. These mixtures have a considerably reduced cold flow but are completely unsuitable, or can be used only to a limited extent, for certain applications, such as the covering of holes in base materials (the so-called "tenting" technique). The tenting technique is used with increasing frequency for the production of printed circuit boards. When processed by the copper clad technique, the drilled and brushed boards are chemically metallized in the holes and then reinforced over the entire surface, to the required thickness of the final layer, in an electrolytic copper-plating bath. The board is thereafter laminated with a photoresist and then exposed to actinic radiation. During this procedure, the resist layer is cured in the areas of the wiring paths and solder pads. The holes are covered on both sides by cured resist areas, which remain during development of the layer. The bared areas of the copper layer are etched away.
The tenting technique is also important for the covering of so-called holding or position holes. These are holes of up to 6 mm or more in diameter which are provided in the boards to permit handling during various treatment steps. In accordance with the semiadditive method for the production of printed circuit boards, metal is deposited electrolytically in those areas not covered by the resist template. During this procedure, the position holes must also be covered, since otherwise they would be narrowed by metal deposition and would thereby be rendered useless for further processing. The resist template is then removed, and the conductive copper underneath is etched away with an etching agent, the copper present in the holding holes and position holes also being removed.
The photoresist thus has to meet strict requirements in the tenting technique. In the exposed state, it must be flexible and resistant to etching and plating baths, and must be self-supporting in covering holes of 6 mm and more.
German Auslegeschrift No. 2,517,656 describes other mixtures, similar to those described above, that contain, as a binder, a combination of two acidic polymers. Under certain conditions, one of these polymers is soluble in dilute aqueous sodium hydroxide solution and the other is insoluble. The methacrylic acid copolymers described therein are insoluble in the aqueous sodium hydroxide solution.